Separations of a gas from admixture with other gases are important industrial processes. In such processes the objective may be either to obtain a product gas enhanced in a particular gas or a product from which that particular gas has an undesired constituent removed therefrom. For example, there are commercial scale processes to separate air to obtain nitrogen, oxygen, and argon and for air prepurification.
Air separation can be accomplished using adsorption processes, in particular, pressure swing (PSA) and vacuum pressure swing types (VPSA). In PSA and VPSA processes, compressed air is pumped through a fixed bed of an adsorbent exhibiting an adsorptive preference for one of the main constituents whereby an effluent product stream enhanced in the non-adsorbed (or lesser adsorbed) constituent is obtained. Compared to cryogenic processes, adsorption processes for air separation require relatively simple equipment and are relatively easy to maintain. Adsorption processes, however, typically have lower product recovery than many cryogenic processes. For this reason, improvements in the adsorption processes remain important goals. One principal means of improvement is the discovery and development of better adsorbents.
One way to improve adsorption is to enhance the mass transfer rate of adsorbent materials, particularly those used in PSA and VPSA. With a fast mass transfer rate, one can have short cycle time and low power consumption and therefore high adsorbent productivity and high process efficiency in PSA/VPSA systems and processes. It has been recognized that it is possible to shorten cycle time by reducing particle size of adsorbent aggregates. This recognition has been based upon the assumption that the time needed for adsorbates to travel through the macropores of the agglomerated adsorbent particles limits the adsorption/desorption cycle time, i.e., macropore diffusion is the rate limiting step in adsorption processes. It has also been recognized that increased porosity will improve macropore diffusion and decreasing binder content may result in increased porosity. However, it is also desirable to use an adsorbent with high crush strength. Too little binder may result in a weak adsorbent particle that will collapse in the bed and too much binder will strengthen the adsorbent particle, but may create a particle that is too dense, resulting in a poor mass transfer rate.
There are many adsorbent compositions and manufacturing processes known in the art for air separation processes and/or for hydrocarbon processing. For example, U.S. Pat. No. 5,053,374 (Absil et al.) describes the use of a colloidal binder to prepare extrudates of zeolite materials for catalysis applications including hydrocarbon processing. Absil et al. define “zeolite” as porous crystalline silicates that contain silicon and oxygen as predominant framework atoms and other components such as aluminum are present in minor amounts. Furthermore, the colloidal binder is preferably a low acidity refractory oxide (with oxides of silicon, titanium, germanium and zirconium being most useful), which functions to lower the acidity of the catalyst and reduce coking in hydrocarbon conversion processes. Absil et al. teach making extrudates with high binder content (e.g., 35 wt % silica) in combination with screw extrusion to produce dense particles with high levels of compaction.
U.S. Pat. No. 3,296,151 (Heinze et al.) describes zeolite granules bonded with silica and made by a sol-gel conversion process designed to yield spherical particles. The molecular sieve content should be between 50-90% and the concentration of MgO (used in the granulation process as a gelling agent) should be between 0.1-3%. Heine et al. teach sol-gel conversion and bead formation from droplets.
U.S. Pat. No. 5,120,693 (Connolly et al.) describes agglomerates of molecular sieve in the size range 40-800 μm, which are formed using a silica bonding agent, wherein the silica particles are in the range 5-20 nm. A spray drying technique is used to make these small adsorbent particles and these molecular sieves are classified as high silica molecular sieves and have SiO2/Al2O3 ratios greater than 18.
U.S. Pat. No. 5,948,726 (Moskovitz et al.) discloses adsorbents and/or catalysts that are bound with colloidal oxides in the presence of acid which is a necessary component of the formulation, and serves to cross-link the binder with the adsorbent or catalyst component. The use of an acid to cross-link the binder to the adsorbent may be detrimental to the performance of low silica zeolite adsorbents characterized by SiO2/Al2O3 ratios of less than or equal to 3, which possess lower stabilities than the higher silica zeolites taught by Moskovitz et al. Acids of the types deemed suitable by Moskovitz et al. can damage or destroy low silica zeolites.
Increased binder content provides higher crush strength, but may cause matting and inhomogeneity in the adsorbent particles. This can lead to lower mass transfer rates and inconsistent adsorbent particles. There remains a need for adsorbents with both high crush strength and high mass transfer rate and processes for consistently making such adsorbents.